A Japanese form of collaborative poetry brought to its highest form by Basho. Renku, also known as haikai no renga is over one thousand years old, and it is usually produced by a group of poets. There are different sub-forms, with varying numbers of verses and stanzas. Traditions specifies in great detail the typographical layout of the poem; also, some specific verses must be about certain themes (the moon, blossoms...).
The renku is a highly complex and refined form, still very much alive in Japan. Read more about it at: http://renku.home.att.net/, the Home of the Renku.